Why was I sent an email from LinkedIn asking if I knew this person?

[Elijah]

LinkedIn sent me an email the other day, asking me if I knew my ex boyfriend. I find this weird because I haven't talked to him in years or even attempted to look him up on any website. Was I sent this because he recommended I connect with him on this website? Or was he typing my name in to find me? I'm new to website so I'm not sure.

[DunbarPappy®ϟϟ]

"LinkedIn" like many other sites have default settings that are very liberal and often intrusive, when it comes to sharing your info with other people, even strangers.

Better go there, log in, and go to your Account settings (or similar) and see exactly what's going on. Tighten up anything that remotely looks like a leakage of your information.

Never take for granted that their perceptions of "it's OK to do this" meets your standards...in fact, you should assume the opposite.
(And you can't exclude the possibility that this was some kind of e-mail phishing attempt not even from LinkedIn, for God knows what reason. Use your normal links/methods to connect to them. Do NOT use any links furnished in that e-mail.)
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Lastly, cross-site leakage of contacts & other info is now the normal revenue generating model.
You'll need to examine all your accounts "Privacy" or "Sharing" settings to make sure you don't get blasted across services.
Facebook is notorious about this kind of action, but many other sites see one site doing something new, so they adopt the same behavior. Always be on the look out for changes to the original Terms you'd signed up for, and terminate the things you don't agree with.